Steering control device

ABSTRACT

The turning amount of a turning unit is controlled according to the steering amount of a steering unit mechanically uncoupled from the turning unit. A steering reaction force characteristic in which the larger the self-aligning torque, the larger the steering reaction force is set on coordinates with the self-aligning torque and the steering reaction force as coordinate axes. When the steering reaction force corresponding to the self-aligning torque is applied to the steering unit based on the steering reaction force characteristic, the steering reaction force characteristic is offset on the coordinates in such a direction that the closer the lateral position of an own vehicle to a white line, the larger the absolute value of the steering reaction force.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationNos. 2012-210290, filed Sep. 25, 2012 and 2012-212181, filed on Sep. 26,2012, each incorporated herein in its entirety.

TECHNICAL FIELD

The present invention relates to a steering control device.

BACKGROUND

Japanese Patent Application Publication No. 2010-188825 discloses anelectric power steering system configured to apply an assist torqueaccording to a sign of a steering torque inputted to a steering wheel bya driver.

However, in the above conventional technology, a steering angle neutralposition and a steering torque neutral position coincide with eachother. For this reason, when a driver performs corrective steering overthe steering angle neutral position to return the vehicle running on oneside of the lane back to the center of the lane, the sign of thesteering torque is inverted and the direction of controlling the forceby the driver is switched accordingly. Thus, there has been a problem ofa large steering load on the driver.

SUMMARY

It is an object of the present invention to provide a steering controldevice capable of reducing steering load when a driver performscorrective steering to return a vehicle running on one side of a laneback to the center of the lane.

According to the present invention, a turning amount of a turning unitis controlled according to a steering amount of a steering unitmechanically uncoupled from the turning unit. A steering reaction forcecharacteristic in which the larger the self-aligning torque, the largerthe steering reaction force is set on coordinates with the self-aligningtorque and the steering reaction force as coordinate axes. When asteering reaction force corresponding to the self-aligning torque isapplied to the steering unit based on the steering reaction forcecharacteristic, the steering reaction force characteristic is offset onthe coordinates in such a direction that the closer the lateral positionof the own vehicle to the white line, the larger the absolute value ofthe steering reaction force.

Thus, since the steering torque neutral position is offset more to theturning side than the steering angle neutral position, inversion of thesign of the steering torque is suppressed during the correctivesteering. As a result, the direction of controlling the force by thedriver becomes less likely to be switched. Therefore, the steering loadon the driver can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system diagram showing a steering system of a vehicleaccording to Example 1.

FIG. 2 is a control block diagram of a turning control unit 19.

FIG. 3 is a control block diagram of a steering reaction force controlunit 20.

FIG. 4 is a control block diagram of a disturbance suppression commandturning angle calculation section 32.

FIG. 5 is a control block diagram of a repulsion force calculation part37 according to a yaw angle.

FIG. 6 is a control block diagram of a repulsion force calculation part38 according to a lateral position.

FIG. 7 is a diagram showing control regions of yaw angle F/B control andlateral position F/B control.

FIG. 8 is a time chart showing changes in the yaw angle when the vehicledriving on a straight road of a highway receives single crosswind.

FIG. 9 is a time chart showing changes in the yaw angle and lateralposition when the lateral position F/B control is not performed in thecase where the vehicle driving on a straight road of a highway receivescontinuous crosswind.

FIG. 10 is a time chart showing changes in the yaw angle and lateralposition when the lateral position F/B control is performed in the casewhere the vehicle driving on a straight road of a highway receivescontinuous crosswind.

FIG. 11 is a control block diagram of a lateral force offset section 34.

FIG. 12 is a diagram showing a state where steering reaction forcecharacteristics indicating steering reaction force torque correspondingto a self-aligning torque are offset in the same direction as theself-aligning torque.

FIG. 13 is a characteristic diagram showing a relationship between asteering angle of a steering wheel and a steering torque of a driver.

FIG. 14 is a diagram showing a state where characteristics indicating arelationship between the steering angle of the steering wheel and thesteering torque of the driver are changed by offsetting the steeringreaction force characteristics indicating the steering reaction forcetorque corresponding to the self-aligning torque in the same directionas the self-aligning torque.

FIG. 15 is a control block diagram of a steering reaction force torqueoffset section 36.

FIG. 16 is a control block diagram of a reaction force calculation part39 according to deviation margin time.

FIG. 17 is a control block diagram of a reaction force calculation part40 according to the lateral position.

FIG. 18 is a diagram showing a state where the steering reaction forcecharacteristics indicating the steering reaction force torquecorresponding to the self-aligning torque are offset in a direction inwhich the absolute value of the steering reaction force torque isincreased.

FIG. 19 is a characteristic diagram showing a relationship between thesteering angle of the steering wheel and the steering torque of thedriver.

FIG. 20 is a diagram showing a state where characteristics indicating arelationship between the steering angle of the steering wheel and thesteering torque of the driver are changed by offsetting the steeringreaction force characteristics indicating the steering reaction forcetorque corresponding to the self-aligning torque in the direction inwhich the absolute value of the steering reaction force torque isincreased.

FIG. 21 is a system diagram showing a steering system of a vehicleaccording to Example 2.

FIG. 22 is a control block diagram of an assist torque control unit 28.

FIG. 23 is a control block diagram of an assist torque offset section42.

FIG. 24 is a diagram showing a state where an assist torquecharacteristic indicating an assist torque corresponding to a steeringtorque is offset in a direction in which the absolute value of theassist torque is reduced.

DESCRIPTION OF EMBODIMENTS Example 1 System Configuration

FIG. 1 is a system diagram showing a steering system of a vehicleaccording to Example 1. A steering device according to Example 1 mainlyincludes a steering unit 1, a turning unit 2, a backup clutch 3 and anSBW controller 4. The steering device adopts a steer-by-wire (SBW)system in which the steering unit 1 configured to receive a steeringinput from a driver is mechanically uncoupled from the turning unit 2configured to turn left and right front wheels (turning wheels) 5FL and5FR.

The steering unit 1 includes a steering wheel 6, a column shaft 7, areaction force motor 8 and a steering angle sensor 9. The column shaft 7rotates integrally with the steering wheel 6. The reaction force motor 8is a brushless motor, for example, and is a coaxial motor whose outputshaft is coaxial with the column shaft 7. The reaction force motor 8outputs a steering reaction force torque to the column shaft 7 inresponse to a command from the SBW controller 4. The steering anglesensor 9 detects an absolute rotation angle of the column shaft 7, i.e.,a steering angle of the steering wheel 6.

The turning unit 2 includes a pinion shaft 11, a steering gear 12, aturning motor 13 and a turning angle sensor 14. The steering gear 12 isa rack and pinion type steering gear, and turns the front wheels 5L and5R according to rotation of the pinion shaft 11. The turning motor 13 isa brushless motor, for example, and has an output shaft coupled to arack gear 15 through an unillustrated reducer. The turning motor 13outputs a turning torque to turn the front wheels 5 to a rack 16 inresponse to a command from the SBW controller 4.

The turning angle sensor 14 detects an absolute rotation angle of theturning motor 13. Here, the rotation angle of the turning motor 13 andthe turning angle of the front wheels 5 always have a uniquely definedcorrelation with each other. Thus, the turning angle of the front wheels5 can be detected from the rotation angle of the turning motor 13.Hereinafter, unless otherwise described, it is assumed that the turningangle of the front wheels 5 is calculated from the rotation angle of theturning motor 13.

The backup clutch 3 is provided between the column shaft 7 in thesteering unit 1 and the pinion shaft 11 in the turning unit 2. Thebackup clutch 3 mechanically uncouples the steering unit 1 from theturning unit 2 when released, and mechanically couple the steering unit1 to the turning unit 2 when engaged.

In addition to the steering angle sensor 9 and the turning angle sensor14, an image of a driving road in front of an own vehicle, which is shotby a camera 17, and a vehicle speed (vehicle body speed) detected by avehicle speed sensor 18 are inputted to the SBW controller 4.

The SBW controller 4 includes: a turning control unit 19 configured tocontrol the turning angle of the front wheels 5FL and 5FR; a steeringreaction force control unit 20 configured to control the steeringreaction force torque to be applied to the column shaft 7; and an imageprocessing unit 21. The turning control unit 19 generates a commandturning angle based on certain kinds of input information, and outputsthe generated command turning angle to a current driver 22.

The current driver 22 controls a command current to the turning motor 13by angle feedback to cause an actual turning angle detected by theturning angle sensor 14 to coincide with the command turning angle. Thesteering reaction force control unit 20 generates a command steeringreaction force torque based on certain kinds of input information, andoutputs the generated command steering reaction force torque to acurrent driver 23.

The current driver 23 controls a command current to the reaction forcemotor 8 by torque feedback to cause an actual steering reaction forcetorque estimated from a current value of the reaction force motor 8 tocoincide with the command steering reaction force torque.

The image processing unit 21 recognizes white lines (driving lanemarking) on the left and right of a driving lane by performing imageprocessing, such as edge extraction, on the image of the driving road infront of the own vehicle shot by the camera 17.

In addition, in case of a failure of the SBW system, the SBW controller4 mechanically couples the steering unit 1 to the turning unit 2 byengaging the backup clutch 3, thereby enabling the rack 16 to be movedin an axial direction by steering of the steering wheel 6. In thisevent, control may be performed, which is equivalent to an electricpower steering system to assist steering force of a driver with anassist torque of the turning motor 13.

The above SBW system may be configured as a redundant system includingmultiple sensors, controllers and motors. Moreover, the turning controlunit 19 and the steering reaction force control unit 20 may be providedseparately.

In Example 1, stability control and correction steering reductioncontrol are carried out in the aim of reducing a correction steeringamount of the driver and reducing steering load. In the stabilitycontrol, two feedback (F/B) controls are performed for the purpose ofimproving the vehicle stability against disturbances (crosswind, roadsurface unevenness, wheel tracks, road surface cant, and the like).

1. Yaw Angle F/B Control

Correcting the turning angle according to a yaw angle, which is an angleformed by the white line and the traveling direction of the own vehicle,to reduce the yaw angle generated by the disturbance.

2. Lateral Position F/B Control

Correcting the turning angle according to a distance (lateral position)to the white line to reduce a lateral position change that is anintegral value of the yaw angle generated by the disturbance.

In the correction steering reduction control, three reaction forceoffset controls are performed for the purpose of improving the vehiclestability for the steering input by the driver.

1. Reaction Force Offset Control According to Lateral Position

Offsetting steering reaction force characteristics corresponding to aself-aligning torque according to the lateral position in a direction inwhich the absolute value of the steering reaction force is increased tosuppress inversion of the sign of the steering torque when the driverperforms correction steering over a steering angle neutral position.

2. Reaction Force Offset Control According to Deviation Margin Time

Offsetting steering reaction force characteristics corresponding to aself-aligning torque according to deviation margin time (time to reachthe white line) in a direction in which the absolute value of thesteering reaction force is increased to suppress inversion of the signof the steering torque when the driver performs correction steering overa steering angle neutral position.

3. Reaction Force Offset Control According to Curvature

Offsetting steering reaction force characteristics corresponding to aself-aligning torque according to the curvature of the white line in thesame sign direction as the self-aligning torque to reduce maintainedsteering force of the driver during turning and suppress a maintainedangle change relative to a maintained steering force change.

[Turning Control Unit]

FIG. 2 is a control block diagram of the turning control unit 19. An SBWcommand turning angle calculation section 31 calculates a SBW commandturning angle based on the steering angle and the vehicle speed. Adisturbance suppression command turning angle calculation section 32calculates a disturbance suppression command turning angle to correctthe SBW command turning angle in the stability control based on thevehicle speed and the white line information. The disturbancesuppression command turning angle calculation section 32 is described indetail later. An adder 19 a outputs a value obtained by adding up theSBW command turning angle and the disturbance suppression commandturning angle to the current driver 22 as a final command turning angle.

[Steering Reaction Force Control Unit]

FIG. 3 is a control block diagram of the steering reaction force controlunit 20. A lateral force calculation section 33 calculates a tirelateral force, based on the steering angle and the vehicle speed, byreferring to a steering angle-lateral force conversion map showing arelationship between the steering angle and the tire lateral force foreach vehicle speed in a conventional steering device, which is obtainedbeforehand by experiment or the like. The steering angle-lateral forceconversion map has the following characteristics. Specifically, thelarger the steering angle, the larger the tire lateral force. Also, thesmaller the steering angle, the larger the change amount of the tirelateral force relative to the change amount of the steering anglecompared with the case of a larger steering angle. Moreover, the higherthe vehicle speed, the smaller the tire lateral force.

A lateral force offset section 34 calculates a lateral force offsetamount, based on the vehicle speed and the white line information, tooffset the steering reaction force characteristics in the reaction forceoffset control according to the curvature. The lateral force offsetsection 34 is described in detail later. A subtractor 20 a subtracts thelateral force offset amount from the tire lateral force.

A SAT calculation section 35 calculates a steering reaction force torquegenerated by the tire lateral force, based on the vehicle speed and thetire lateral force after offset by the lateral force offset amount, byreferring to a lateral force-steering reaction force torque conversionmap showing a relationship between the tire lateral force and thesteering reaction force torque in a conventional steering device, whichis obtained beforehand by experiment or the like. The tire lateralforce-steering reaction force torque conversion map has the followingcharacteristics. Specifically, the larger the tire lateral force, thelarger the steering reaction force torque. Also, the smaller the tirelateral force, the larger the change amount of the steering reactionforce torque relative to the change amount of the tire lateral forcecompared with the case of a larger tire lateral force. Moreover, thehigher the vehicle speed, the smaller the steering reaction forcetorque. These characteristics are obtained by simulating the reactionforce generated in the steering wheel by the self-aligning torque thatreturns the wheels to their straight-driving state, which is generatedby a road surface reaction force, in a conventional steering device.

An adder 20 b adds up the steering reaction force torque and steeringreaction force torque components (spring term, viscosity term andinertia term) corresponding to the steering characteristics. The springterm is a component proportional to the steering angle, and iscalculated by multiplying the steering angle by a predetermined gain.The viscosity term is a component proportional to the steering anglespeed, and is calculated by multiplying the steering angle speed by apredetermined gain. The inertia term is a component proportional to thesteering angle acceleration, and is calculated by multiplying thesteering angle acceleration by a predetermined gain.

A steering reaction force torque offset section 36 calculates a steeringreaction force torque offset amount to offset the steering reactionforce characteristics in the reaction force offset control according tothe lateral position or the deviation margin time, based on the vehiclespeed and the image of the driving road in front of the own vehicle. Thesteering reaction force torque offset section 36 is described in detaillater.

An adder 20 c outputs a value obtained by adding up the steering torqueoffset amount and the steering reaction force torque added with thesteering reaction force torque components corresponding to the steeringcharacteristics to the current driver 23 as a final command steeringreaction force torque.

[Disturbance Suppression Command Turning Angle Calculation Section]

FIG. 4 is a control block diagram of the disturbance suppression commandturning angle calculation section 32. A yaw angle calculation part 32 acalculates a yaw angle that is an angle formed by the white line at afront gaze point and the traveling direction of the own vehicle. It isassumed that the yaw angle at the front gaze point is an angle formed bythe white line and the vehicle traveling direction after a predeterminedtime period (e.g., 0.5 seconds). The yaw angle can be easily andaccurately detected by calculating the yaw angle based on the image ofthe driving road shot by the camera 17.

A curvature calculation part 32 b calculates the curvature of the whiteline at the front gaze point. A lateral position calculation part 32 ccalculates a distance to the white line at the front gaze point. Arepulsion force calculation part 37 according to the yaw anglecalculates a repulsion force of the vehicle to reduce the yaw anglegenerated by the disturbance in the yaw angle F/B control, based on theyaw angle, the curvature and the vehicle speed. The repulsion forcecalculation part 37 according to the yaw angle is described in detaillater.

A repulsion force calculation part 38 according to the lateral positioncalculates a repulsion force of the vehicle to reduce the lateralposition change generated by the disturbance in the lateral position F/Bcontrol, based on the yaw angle, the curvature, the vehicle speed andthe distance to the white line at the front gaze point. The repulsionforce calculation part 38 according to the lateral position is describedin detail later. An adder 32 d calculates a lateral-direction repulsionforce by adding up the repulsion force corresponding to the yaw angleand the repulsion force corresponding to the lateral position.

A target yaw moment calculation part 32 e calculates a target yaw momentbased on the lateral-direction repulsion force, a wheel base (inter-axledistance), a rear-wheel axle load, and a front-wheel axle load. To bemore specific, a value obtained by multiplying the lateral-directionrepulsion force by a ratio of the rear-wheel axle load to the vehicleweight (front-wheel axle load+rear-wheel axle load) and the wheel baseis used as the target yaw moment. A target yaw acceleration calculationpart 32 f calculates a target yaw acceleration by multiplying the targetyaw moment by a yaw inertia moment coefficient. A target yaw ratecalculation part 32 g calculates a target yaw rate by multiplying thetarget yaw acceleration by a headway.

A command turning angle calculation part 32 h calculates a disturbancesuppression command turning angle δ_(st)* by referring to the followingequation based on the target yaw rate φ*, the wheel base WHEEL_BASE, thevehicle speed V and the vehicle characteristic speed vCh. Here, thevehicle characteristic speed V_(ch) is a parameter in the known“Ackermann's formula”, and represents self-steering characteristics ofthe vehicle.δ_(st)*=(φ*×WHEEL_BASE×(1+(V/vCh)²)×180)/(V×M_PI)

Note that M_PI is a predetermined coefficient.

A limiter processing part 32 i limits the maximum value of thedisturbance suppression command turning angle δ_(st)* and the upperlimit of the change rate. The maximum value is a turning angle range(e.g., 0.2° on the left and right sides) of the front wheels 5FL and 5FRcorresponding to a range of an allowance around the neutral positionwhen the steering angle of the steering wheel 6 is within an angle range(e.g., 3° on the left and right sides) of the allowance, in aconventional steering device (in which the steering unit is mechanicallyconnected with the turning unit).

FIG. 5 is a control block diagram of the repulsion force calculationpart 37 according to the yaw angle. An upper and lower limiter 37 aperforms upper and lower limitation processing on the yaw angle. Whenthe yaw angle is a positive value (the yaw angle is positive when thewhite line intersects with the extension of the vehicle travelingdirection), the upper and lower limiter sets a value (e.g., 1°) that isnot less than a predetermined value that enables suppression of thedisturbance and less than a value that causes the vehicle to vibrate anda value generated by steering by the driver. On the other hand, when theyaw angle is a negative value, the upper and lower limiter sets 0.

A yaw angle F/B gain multiplication part 37 b multiplies the yaw angleafter the limitation processing by a yaw angle F/B gain. The yaw angleF/B gain is not less than a predetermined value that enablesresponsiveness to be ensured while avoiding insufficient control amount,and less than a value that causes the vehicle to vibrate and a value atwhich the driver feels a shift in the neutral position between thesteering angle and the turning angle.

A vehicle speed correction gain multiplication part 37 c multiplies thevehicle speed by a vehicle speed correction gain. The vehicle speedcorrection gain has characteristics of reaching the maximum value withina range of 0 to 70 km/h, gradually decreasing within a range of 70 to130 km/h, and reaching the minimum value (0) at 130 km/h or more.

A curvature correction gain multiplication part 37 d multiplies thecurvature by a curvature correction gain. The curvature correction gainhas characteristics that the larger the curvature, the smaller thecurvature correction gain, and sets the upper limit and lower limit (0).A multiplier 37 e obtains a repulsion force corresponding to the yawangle by multiplying outputs from the yaw angle F/B gain multiplicationpart 37 b, the vehicle speed correction gain multiplication part 37 cand the curvature correction gain multiplication part 37 d.

FIG. 6 is a control block diagram of the repulsion force calculationpart 38 according to the lateral position. A subtractor 38 a obtains alateral position deviation by subtracting the distance to the white lineat the front gaze point from a preset lateral position threshold (e.g.,90 cm). An upper and lower limiter 38 b performs upper and lowerlimitation processing on the lateral position deviation. The upper andlower limiter takes a predetermined positive value when the lateralposition deviation is a positive value, and takes 0 when the lateralposition deviation is a negative value.

A distance correction gain multiplication part 38 c multiplies thedistance to the white line at the front gaze point by a distancecorrection gain. The distance correction gain has the followingcharacteristics, and sets the lower limit. Specifically, the distancecorrection gain takes its maximum value when the distance to the whiteline is a predetermined value or less. Moreover, when the distance tothe white line exceeds the predetermined value, the longer the distance,the smaller the distance correction gain.

A lateral position F/B gain multiplication part 38 d multiplies thedistance to the white line after the correction by the distancecorrection gain multiplication part 38 c by a lateral position F/B gain.The lateral position F/B gain is not less than a predetermined valuethat enables responsiveness to be ensured while avoiding insufficientcontrol amount, and less than a value that causes the vehicle to vibrateand a value at which the driver feels a shift in the neutral position.Furthermore, the lateral position F/B gain is set to a value smallerthan the yaw angle F/B gain obtained by the yaw angle F/B gaincalculation part 37 b.

A vehicle speed correction gain multiplication part 38 e multiplies thevehicle speed by a vehicle speed correction gain. The vehicle speedcorrection gain has characteristics of reaching the maximum value withina range of 0 to 70 km/h, gradually decreasing within a range of 70 to130 km/h, and reaching the minimum value (0) at 130 km/h or more. Acurvature correction gain multiplication part 38 f multiplies thecurvature by a curvature correction gain. The curvature correction gainhas characteristics that the larger the curvature, the smaller thecurvature correction gain, and sets the upper limit and lower limit (0).A multiplier 38 g obtains a repulsion force corresponding to the lateralposition by multiplying outputs from the lateral position F/B gainmultiplication part 38 d, the vehicle speed correction gainmultiplication part 38 e and the curvature correction gainmultiplication part 38 f.

[Stability Control Operation]

In Example 1, as the stability control, the yaw angle F/B control toreduce the yaw angle generated by the disturbance and the lateralposition F/B control to reduce the lateral position change that is anintegral value of the yaw angle generated by the disturbance areperformed. The yaw angle F/B control is performed when the yaw angle isgenerated regardless of the lateral position. The lateral position F/Bcontrol is performed when the distance to the white line reaches apredetermined lateral position threshold (90 cm) or less. Morespecifically, the vicinity of the center of the driving lane is set as adead zone of the lateral position F/B control. FIG. 7 shows controlregions of the both F/B controls. Here, φ is the yaw angle.

FIG. 8 is a time chart showing changes in the yaw angle when the vehicledriving on a straight road of a highway receives single crosswind. It isassumed that the vehicle is driving near the center of the driving lane.When the vehicle receives single crosswind and a yaw angle is generated,the yaw angle F/B control is performed, in which a repulsion forcecorresponding to the yaw angle is calculated, a disturbance suppressioncommand turning angle is calculated to obtain the repulsion force, andthe SBW command turning angle based on the steering angle and thevehicle speed is corrected.

When the vehicle is driven along the driving lane, the direction of thewhite lines coincides with the vehicle traveling direction particularlyon the straight road. Thus, the yaw angle becomes zero. Morespecifically, in the yaw angle F/B control according to Example 1, thegenerated yaw angle is considered to be one generated by thedisturbance. Therefore, by reducing the yaw angle, the vehicle stabilityagainst the disturbance particularly during the straight-driving statecan be improved. As a result, a correction steering amount by the drivercan be reduced.

As a technology of suppressing the influence of disturbance such ascrosswind on vehicle behavior, there has heretofore been known atechnology of applying a turning torque for suppressing the disturbanceto a steering system in a conventional steering device. As for the SBWsystem, there has been known a technology of applying a steeringreaction force component that encourages turning for suppressing thedisturbance to the steering wheel. However, in such conventionalsteering devices, variations in the steering reaction force cause thedriver to feel discomfort.

On the other hand, the stability control including the yaw angle F/Bcontrol according to Example 1 focuses on the point that the steeringwheel 6 and the front wheels 5L and 5R can be controlled independentlyof each other, which is one of the characteristics of the SBW system inwhich the steering wheel 6 and the front wheels 5L and 5R aremechanically uncoupled from each other. In the stability control, theturning angle of the front wheels 5L and 5R is controlled based on thecommand turning angle obtained by adding up the SBW command turningangle corresponding to the steering angle and the vehicle speed and thedisturbance suppression command turning angle corresponding to the yawangle. Moreover, the tire lateral force is estimated based on thesteering angle and the vehicle speed, and the steering reaction force iscontrolled based on the command steering reaction force corresponding tothe estimated tire lateral force and the vehicle speed.

More specifically, since the disturbance suppression turning angle isapplied directly to the front wheels 5L and 5R, it is no longer requiredto apply the steering reaction force component that encourages turningto suppress the disturbance. Furthermore, by applying the steeringreaction force corresponding to the tire lateral force estimated fromthe steering angle, variations in the tire lateral force caused by theturning to suppress the disturbance are not reflected on the steeringreaction force. Thus, the sense of discomfort experienced by the drivercan be reduced. In the conventional SBW system, the tire lateral forceis estimated from a rack axial force detected by a sensor or the turningangle, and a steering reaction force corresponding to the estimated tirelateral force is applied. Therefore, variations in the tire lateralforce caused by the turning to suppress the disturbance are alwaysreflected on the steering reaction force, resulting in a sense ofdiscomfort experienced by the driver. Meanwhile, in Example 1, only thetire lateral force caused by steering by the driver is reflected on thesteering reaction force, and the turning for disturbance suppressioncauses no variations in the steering reaction force. Thus, the sense ofdiscomfort experienced by the driver can be reduced.

Here, when the disturbance suppression turning angle is applied directlyto the front wheels 5L and 5R, a shift in the neutral position betweenthe steering angle and the turning angle becomes a problem. However, inExample 1, the disturbance suppression command turning angle is setwithin the turning angle range (0.2° on the left and right sides) of thefront wheels 5FL and 5FR corresponding to the range of the allowancearound the neutral position of the steering angle when the steeringwheel 6 is located within the angle range (3° on the left and rightsides) of the allowance, in a conventional steering device. Thegeneration of the yaw angle by the disturbance is more significant inthe straight-driving state than in the turning state. In thestraight-driving state, the steering angle is located in the vicinity ofthe neutral position of the steering angle. More specifically, in manycases, the correction of the turning angle by the yaw angle F/B controlis performed in the vicinity of the steering angle neutral position.Therefore, the neutral position shift amount between the steering angleand the turning angle associated with the application of the disturbancesuppression command turning angle is suppressed within the range of theallowance of steering. Thus, a sense of discomfort caused by the shiftin the neutral position can be suppressed.

Moreover, since the disturbance suppression command turning angle islimited within the range of 0.2° on the left and right sides, the drivercan change the vehicle traveling direction to a desired direction bysteering input even during the stability control. More specifically, thecorrection amount of the turning angle by the disturbance suppressioncommand turning angle is minute compared with the change amount in theturning angle caused by the steering input by the driver. Thus, thevehicle stability against the disturbance can be improved withouthindering the steering by the driver.

As control of lateral movement of the vehicle, there have heretoforebeen known lane departure prevention control in which a yaw moment toprevent the vehicle from departing from the lane is applied to thevehicle when it is detected that the vehicle has a tendency to departfrom the driving lane as well as lane keep control in which a yaw momentis applied to the vehicle so that the vehicle travels near the center ofthe driving lane. However, the lane departure prevention control iscontrol having a control intervention threshold, and is not initiatednear the center of the driving lane. Therefore, the vehicle stabilityagainst the disturbance cannot be ensured. Moreover, the thresholdintervenes in the control even when the driver wishes to pull over thevehicle to the side of the driving lane. This causes the driver to feelirritated. Meanwhile, the lane keep control is control having a targetposition (target line), and can ensure the vehicle stability against thedisturbance. However, in the lane keep control, the vehicle cannot bedriven in a line that deviates from the target line. In addition,reduction in force of the driver gripping the steering wheel isdetermined to be a hands-free state, and thus the control is cancelled.Therefore, the driver needs to always grip the steering wheel with acertain amount of force or more. This results in a large steering loadon the driver.

On the other hand, the yaw angle F/B control according to Example 1 hasno control intervention threshold, and thus can realize a seamlesscontrol to always ensure the stability against the disturbance.Furthermore, the yaw angle F/B control has no target position, and thusallows the driver to drive the vehicle in a desired line. In addition,the control is not cancelled even when the steering wheel 6 is lightlygripped. The steering load on the driver can be reduced.

FIG. 9 is a time chart showing changes in the yaw angle and lateralposition when the lateral position F/B control is not performed in thecase where the vehicle driving on a straight road of a highway receivescontinuous crosswind. It is assumed that the vehicle is driving near thecenter of the driving lane. When the vehicle receives continuouscrosswind and a yaw angle is generated, the yaw angle is reduced by theyaw angle F/B control, but the vehicle strays to the side due to thecontinuous disturbance. This is because the yaw angle F/B control is toreduce the yaw angle, and the turning angle is not corrected when theyaw angle is zero. Therefore, the lateral position change that is theintegral value of the yaw angle generated by the disturbance cannot bedirectly reduced. Note that the lateral position change can beindirectly suppressed (increase in the integral value of the yaw anglecan be suppressed) by setting a large value as the repulsion forcecorresponding to the yaw angle. However, the maximum value of thedisturbance suppression command turning angle is limited to 0.2° on theleft and right sides so as not to cause the driver to feel discomfort.Thus, it is difficult to effectively prevent the vehicle from strayingto the side only by performing the yaw angle F/B control. Furthermore,the yaw angle F/B gain to obtain the repulsion force corresponding tothe yaw angle requires convergence of the yaw angles before the drivernotices changes in the yaw angle, and thus takes a maximum value, whichon the other hand, causes the vehicle to vibrate as it is. Therefore,the yaw angle by which the yaw angle F/B gain is multiplied is limitedto the upper limit (1°) or less by the upper and lower limiter 37 a.More specifically, the repulsion force corresponding to the yaw angle isa repulsion force corresponding to a yaw angle smaller than an actualyaw angle. This can also explain that it is difficult to effectivelyprevent the vehicle from straying to the side only by performing the yawangle F/B control.

Therefore, in the stability control according to Example 1, the lateralposition F/B control is introduced to prevent the vehicle from strayingto the side due to steady disturbance. FIG. 10 is a time chart showingchanges in the yaw angle and lateral position when the lateral positionF/B control is performed in the case where the vehicle driving on astraight road of a highway receives continuous crosswind. In the lateralposition F/B control, the vehicle driving near the center of the drivinglane receives continuous crosswind and strays to the side, and arepulsion force corresponding to a lateral position change (≈yaw angleintegral value) is calculated when the distance to the white linebecomes a lateral position threshold or less. The disturbancesuppression command turning angle calculation section 32 corrects theSBW command turning angle by calculating the disturbance suppressioncommand turning angle based on the lateral direction repulsion forceobtained by adding up the repulsion force corresponding to the lateralposition and the repulsion force corresponding to the yaw angle. Morespecifically, in the lateral position F/B control, the SBW commandturning angle is corrected using the disturbance suppression commandturning angle corresponding to the lateral position. Thus, the lateralposition change caused by the steady disturbance can be directlyreduced, and the vehicle can be prevented from straying to the side. Inother words, the vehicle driving position, for which the yaw angle F/Bcontrol is performed, can be returned to the vicinity of the center ofthe driving lane, which is the dead zone of the lateral position F/Bcontrol.

As described above, the stability control according to Example 1 canimprove the vehicle stability against both transient and steadydisturbances by performing the yaw angle F/B control to reduce the yawangle change caused by the transient disturbance and performing thelateral position F/B control to reduce the yaw angle integral value(lateral position change) caused by the steady disturbance.

Furthermore, in the stability control according to Example 1, thevehicle behavior caused by the control (application of the disturbancesuppression command turning angle) is limited to an extent not noticedby the driver and to an extent not inhibiting a vehicle behavior changecaused by steering by the driver. Also, a change in self-aligning torquecaused by the control is not reflected on the steering reaction force.Thus, the stability control can be performed without making the driveraware of the stability control being executed. As a result, the behaviorof the vehicle having vehicle body specifications as if the vehicle hasexcellent stability against disturbance can be simulated.

Note that the lateral position F/B gain to obtain the repulsion forcecorresponding to the lateral position in the lateral position F/Bcontrol is set to have a value smaller than the yaw angle F/B gain. Asdescribed above, the yaw angle F/B control is required to have highresponsiveness because of the need to converge the yaw angles before thedriver perceives the yaw angle change caused by transient disturbance.On the other hand, the lateral position F/B control is required to stopan increase in the lateral position change, and it takes time for thelateral position to change due to accumulation of the yaw angle integralvalues. Thus, the lateral position F/B control does not needresponsiveness as high as that of the yaw angle F/B control. Inaddition, if the lateral position F/B gain is increased, the controlamount varies significantly depending on the degree of the disturbance,causing the driver to feel discomfort.

[Lateral Force Offset Section]

FIG. 11 is a control block diagram of the lateral force offset section34. A curvature calculation part 34 a calculates a curvature of thewhite line at the front gaze point. An upper and lower limiter 34 bperforms upper and lower limitation processing on the vehicle speed. ASAT gain calculation part 34 c calculates a SAT gain corresponding tothe vehicle speed, based on the vehicle speed after the limitationprocessing. The SAT gain has characteristics that the higher the vehiclespeed, the larger the gain, and sets the upper limit. A multiplier 34 dobtains a lateral force offset amount by multiplying the SAT gain by thecurvature. A limiter processing part 34 e limits the maximum value ofthe lateral force offset amount and the upper limit of the change rate.For example, the maximum value is 1,000 N and the upper limit of thechange rate is 600 N/s.

[Reaction Force Offset Control Operation According to Curvature]

In the reaction force offset control according to the curvature, thelateral force offset amount that increases with an increase in thecurvature of the white line is obtained, and the obtained lateral forceoffset amount is subtracted from the tire lateral force. Thus, steeringreaction force characteristics representing the steering reaction forcetorque corresponding to the tire lateral force calculated by the SATcalculation section 35, i.e., the steering reaction force torquecorresponding to the self-aligning torque are offset in the same signdirection as the self-aligning torque as the curvature of the white lineis increased, as shown in FIG. 12. Note that FIG. 12 shows the case of aright-hand curve and, in the case of a left-hand curve, the steeringreaction force characteristics are offset in a direction opposite tothat shown in FIG. 12.

Conventionally, in the SBW system in which the steering unit and theturning unit are mechanically uncoupled from each other, steeringreaction force characteristics are set to simulate the steering reactionforce corresponding to the self-aligning torque in the conventionalsteering device, and the steering reaction force is applied to thesteering wheel based on the steering reaction force characteristics. Inthis event, the relationship between the steering angle of the steeringwheel and the steering torque of the driver has characteristic A asshown in FIG. 13. More specifically, the larger the absolute value ofthe steering angle, the larger the absolute value of the steeringtorque. Also, the smaller the absolute value of the steering angle, thelarger the change amount of the steering torque relative to the changeamount of the steering angle compared with the case of a larger absolutevalue.

Here, a case is considered where the driver changes the maintainedsteering torque to correct the course during turning. In FIG. 13, themaintained steering torque is reduced to T₂ from a state where thedriver maintains a steering angle θ₁ with a maintained steering torqueT₁, the steering angle becomes θ₂ and the turning angle of the frontwheels 5L and 5R is reduced by the reduction in the steering angle. Inthis event, according to the steering reaction force characteristics inthe SBW system described above, the larger the curvature of the curve,the larger the change in the steering angle relative to the change inthe maintained steering torque. More specifically, the larger thecurvature of the curve, the higher the sensitivity of the vehicle to thesteering torque. This leads to a problem of difficulty in correcting thecourse.

Meanwhile, in the reaction force offset control according to thecurvature in Example 1, with an increase in the curvature of the whiteline, the steering reaction force characteristics indicating thesteering reaction force torque corresponding to the self-aligning torqueare offset in the same sign direction as the self-aligning torque.Accordingly, the characteristics indicating the relationship between thesteering angle and the steering torque are offset in the same signdirection as the steering angle as shown in FIG. 14, and are changedfrom characteristic A to characteristic B. Thus, the larger thecurvature of the white line, the smaller the change amount in thesteering angle relative to the change amount in the maintained steeringtorque. Even when the driver reduces the maintained steering torque toT4 and the reduction amount ΔT₃₋₄ of the maintained steering torque isthe same as the reduction amount ΔT₁₋₂ in the conventional technologyshown in FIG. 13, the reduction amount Δθ₁₋₄ of the steering anglebecomes smaller than the reduction amount Δθ₁₋₂ in the conventionaltechnology. More specifically, the larger the curvature of the curve,the smaller the change in the steering angle can be relative to thechange in the maintained steering torque. Therefore, the sensitivity ofthe vehicle to the steering torque can be reduced. Thus, the vehiclebehavior change becomes gradual, and the driver can more easily correctthe course. Moreover, the maintained steering torque T₃ (<T₁) tomaintain the steering angle θ₁ can be reduced to be smaller than that inthe conventional technology. Thus, the steering load on the driverduring turning can be reduced.

There has heretofore been known a technology of reducing the slope ofthe steering reaction force characteristics along with the increasedcurvature of the white line, for the purpose of reducing the steeringload on the driver during turning. However, in the conventionaltechnology, the larger the curvature, the larger the change in thesteering angle relative to the change in the maintained steering torque.As a result, the sensitivity of the vehicle to the steering torque isincreased. More specifically, reduction in the steering load on thedriver during turning and facilitation of the course correction can beboth realized by offsetting the steering reaction force characteristicsin the same direction as the self-aligning torque according to thecurvature of the white line.

[Steering Reaction Force Torque Offset Section]

FIG. 15 is a control block diagram of the steering reaction force torqueoffset section 36. A yaw angle calculation part 36 a calculates a yawangle at the front gaze point. The yaw angle can be easily andaccurately detected by calculating the yaw angle based on the image ofthe driving road shot by the camera 17.

A lateral position calculation part 36 b calculates lateral positionsrelative to the left and right white lines at the front gaze point andlateral positions relative to the left and right white lines at thecurrent position. Here, when the own vehicle moves across the white lineto the next driving lane, i.e., when a lane change is made, the lateralposition calculation part 36 b switches between the lateral positionsrelative to the left and right white lines at the current position. Morespecifically, the lateral position relative to the left white linebefore the white line is reached is switched to the lateral positionrelative to the right white line after the white line is reached. Also,the lateral position relative to the right white line before the whiteline is reached is switched to the lateral position relative to the leftwhite line after the white line is reached. Note that, when a lanechange is made to a driving lane having a different lane width, thelateral positions are corrected by multiplying the switched lateralposition by a value W₂/W₁ obtained by dividing a lane width W₂ of thedriving lane after the lane change by a lane width W₁ of the drivinglane before the lane change. Here, lane width information of eachdriving lane is acquired from the navigation system 24.

A reaction force calculation part 39 according to deviation margin timecalculates a reaction force corresponding to deviation margin time basedon the vehicle speed, the yaw angle and the lateral positions relativeto the left and right white lines at the front gaze point. The reactionforce calculation part 39 according to the deviation margin time isdescribed in detail later.

A reaction force calculation part 40 according to the lateral positioncalculates a reaction force corresponding to the lateral position, basedon the lateral positions relative to the left and right white lines atthe current position. The reaction force calculation part 40 accordingto the lateral position is described in detail later.

A reaction force selection part 36 c selects the reaction force having alarger absolute value between the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position, as a steering reaction force torque offset amount. Alimiter processing part 36 d limits the maximum value of the steeringreaction force torque offset amount and the upper limit of the changerate. For example, the maximum value is 2 Nm and the upper limit of thechange rate is 10 Nm/s.

FIG. 16 is a control block diagram of the reaction force calculationpart 39 according to the deviation margin time. A multiplier 39 aobtains a lateral speed of the vehicle by multiplying the yaw angle bythe vehicle speed. A divider 39 b obtains a deviation margin time forthe left white line by dividing the lateral position relative to theleft white line at the front gaze point by the lateral speed. A divider39 c obtains a deviation margin time for the right white line bydividing the lateral position relative to the right white line at thefront gaze point by the lateral speed. A deviation margin time selectionpart 39 d selects the shorter deviation margin time between thedeviation margin times for the left and right white lines, as thedeviation margin time.

A reaction force calculation part 39 e according to the deviation margintime calculates a reaction force corresponding to the deviation margintime, based on the deviation margin time. The reaction forcecorresponding to the deviation margin time is inversely proportional tothe deviation margin time (proportional to the reciprocal of thedeviation margin time), and has characteristics of becoming almost zeroin 3 seconds or more.

FIG. 17 is a control block diagram of the reaction force calculationpart 40 according to the lateral position. A subtractor 40 a obtains alateral position deviation relative to the left lane by subtracting thelateral position relative to the left lane from a preset target leftlateral position (e.g., 90 cm). A subtractor 40 b obtains a lateralposition deviation relative to the right lane by subtracting the lateralposition relative to the right lane from a preset target right lateralposition (e.g., 90 cm). A lateral position deviation selection part 40 cselects the larger lateral position deviation between the lateralposition deviations relative to the left and right lanes. A reactionforce calculation part 40 d according to the lateral position deviationcalculates a reaction force corresponding to the lateral position, basedon the lateral position deviation. The reaction force corresponding tothe lateral position has characteristics that the larger the lateralposition deviation, the larger the reaction force, and sets the upperlimit.

[Reaction Force Offset Control Operation According to Lateral Position]

In the reaction force offset control according to the lateral position,the reaction force corresponding to the lateral position is added, asthe steering reaction force torque offset amount, to the steeringreaction force torque. Accordingly, the steering reaction forcecharacteristics indicating the steering reaction force torquecorresponding to the self-aligning torque are offset in a direction inwhich the absolute value of the steering reaction force torque isincreased with a reduction in the distance to the white line, as shownin FIG. 18. Note that FIG. 18 shows the case where the vehicle is closerto the right lane and, in the case where the vehicle is closer to theleft lane, the steering reaction force characteristics are offset in adirection opposite to that shown in FIG. 18.

Here, a case is considered where, in the conventional steering reactionforce control, a sudden rightward turning operation by the driver shiftsthe vehicle driving position to the right and then the driver performscorrective steering to return the driving position to the vicinity ofthe center of the driving lane. It is assumed that the steering angleand steering torque when the sudden operation is performed by the driverare at the position of point P₁ on characteristic A in FIG. 19.Characteristic A is a characteristic indicating a relationship betweenthe steering angle and the steering torque when the steering reactionforce characteristics are set by simulating a conventional steeringdevice, as in the case of FIG. 13. In order to return the drivingposition to the vicinity of the center of the driving lane from thisstate, the front wheels need to be turned to the left. Thus, the driverperforms a turning back operation to the steering angle neutral positionand then performs a turning operation from the steering angle neutralposition, thereby aligning the steering wheel with the target angle θ₅.In this event, in the conventional technology, the steering angleneutral position (steering angle zero point) coincides with the steeringtorque neutral position (steering torque zero point). Thus, the steeringtorque needs to be reduced up to the steering angle neutral position andthen increased when the driving position exceeds the steering angleneutral position. More specifically, in the case of performingcorrective steering over the steering angle neutral position, the signof the steering torque is inverted, and a direction of controlling theforce by the driver is switched. Moreover, the change amount of thesteering angle relative to the change amount of the steering torque issignificantly small in the vicinity of the steering torque neutralposition compared with the other steering angle regions. Therefore, thesteering load on the driver is large, and it is difficult to control thesteering wheel at the target angle θ₅. Thus, the vehicle drivingposition is likely to be overshot, leading to a problem of increasedcorrective steering amount.

On the other hand, in the reaction force offset control according to thelateral position in Example 1, the steering reaction force torquecorresponding to the self-aligning torque is offset in the direction inwhich the absolute value of the steering reaction force torque isincreased with a shorter distance to the white line. As a result, thecharacteristics indicating a relationship between the steering angle andthe steering torque are offset in a direction in which the absolutevalue of the steering torque is increased, as shown in FIG. 20, andcontinuously changed from characteristic A to characteristic C as thedistance to the white line is reduced. In this event, in order tomaintain the steering angle, the steering torque needs to be increased.If the steering torque is constant, the steering wheel 6 is graduallyreturned to the steering angle neutral position (point P₁ to point P₂).Thus, the vehicle driving position can be prevented from being shiftedto the right by a sudden turning operation by the driver. On the otherhand, when the steering angle is maintained by the driver, the steeringangle and the steering torque are moved from point P₁ to point P₃. Whenthe driver performs corrective steering from this state, the sign of thesteering torque is not inverted until the vehicle driving positionreaches the steering torque neutral position during a turning operationfrom the steering angle neutral position, since the steering torqueneutral position is offset to the turning side than the steering angleneutral position in characteristic C. Therefore, the driver can controlthe turning angle of the front wheels 5L and 5R just by reducing thesteering torque and stopping the rotation of the steering wheel 6 whenthe steering wheel 6 comes to the target angle. More specifically, inthe reaction force offset control according to the lateral position inExample 1, the direction of controlling the force by the driver is notlikely to be switched. Thus, the corrective steering by the driver canbe facilitated. As a result, the vehicle driving position becomes lesslikely to be overshot, and thus the corrective steering amount can bereduced.

There has heretofore been known a technology of increasing a steeringreaction force to such an extent as to come closer to the white line forthe purpose of suppressing a shift in a driving position due to a suddenoperation by a driver. In the conventional technology, the steeringwheel is just increased in weight as the driving position approaches thewhite line. Since the steering torque neutral position in the steeringreaction force characteristics always coincides with the steering angleneutral position, the sign of the steering torque is inverted in thecorrective steering over the steering angle neutral position. As aresult, the steering load on the driver is not reduced. Morespecifically, suppression of the shift in the driving position andreduction in the steering load on the driver can be both realized byoffsetting the steering reaction force torque corresponding to theself-aligning torque in a direction in which the absolute value of thesteering reaction force torque is increased with a shorter distance tothe white line.

In the reaction force offset control according to the lateral positionin Example 1, the shorter the distance to the white line, the larger theoffset amount. Therefore, the steering torque neutral position is offsetto a position more distant from the steering angle neutral position witha shorter distance to the white line. When the driver performscorrective steering to return the vehicle driving position to thevicinity of the center of the driving lane, the turning operation amountfrom the steering angle neutral position needs to be increased as thedriving position is closer to the white line. In this event, when theoffset amount of the steering torque neutral position relative to thesteering angle neutral position is small, there is a possibility thatthe steering torque exceeds the neutral position and the sign of thesteering torque is inverted before the steering wheel comes to thetarget angle. Therefore, the steering torque can be prevented fromexceeding the neutral position by increasing the offset amount with ashorter distance to the white line.

In the reaction force offset control according to the lateral positionin Example 1, the lateral position calculation part 36 b switchesbetween the lateral positions relative to the left and right white linesat the current position, when the own vehicle reaches the white line. Inthe reaction force offset control according to the lateral position, theown vehicle more easily returns to the vicinity of the center of thedriving lane by increasing the steering reaction force as the ownvehicle moves farther from the vicinity of the center of the drivinglane. More specifically, the yaw angle integral value (lateral positionchange) is recognized as the disturbance, and the steering reactionforce is controlled so as to guide the vehicle in a direction in whichthe yaw angle integral value becomes zero. For this reason, when a lanechange is made, the yaw angle integral value needs to be reset. If theyaw angle integral value is not reset, the steering reaction force toreturn the vehicle to the vicinity of the center of the driving lanebefore the lane change keeps acting even after the lane change. Thisinhibits the operations by the driver. Note that the vehicle cannot beguided to the vicinity of the center of the driving lane after the lanechange only by setting the integral value to zero.

Therefore, in Example 1, when the own vehicle reaches the white line,the operation can be recognized as an intended operation by the driver.In such a case, the lateral positions relative to the left and rightwhite lines at the current position are switched. In other words, byinverting the sign of the yaw angle integral value, the position towhich the own vehicle is guided is changed from the vicinity of thecenter of the driving lane before the lane change to the vicinity of thecenter of the driving lane after the lane change. Thus, the steeringreaction force can be generated to guide the own vehicle to the vicinityof the center of the driving lane after the lane change. In this event,by taking into consideration the ratio W₂/W₁ of the lane width W₂ of thedriving lane after the lane change to the lane width W₁ of the drivinglane before the lane change, an accurate lateral position can be set,and an optimum offset amount for guiding the own vehicle to the vicinityof the center of the driving lane can be set.

[Reaction Force Offset Control Operation According to Deviation MarginTime]

In the reaction force offset control according to the deviation margintime, the reaction force corresponding to the deviation margin time isadded as the steering reaction force torque offset amount to thesteering reaction force torque. Thus, the steering reaction forcecharacteristics indicating the steering reaction force torquecorresponding to the self-aligning torque are offset in a direction inwhich the shorter the deviation margin time, the larger the absolutevalue of the steering reaction force torque, as shown in FIG. 18. Notethat FIG. 18 shows the case where the vehicle is closer to the rightlane and, in the case where the vehicle is closer to the left lane, thesteering reaction force characteristics are offset in a directionopposite to that shown in FIG. 18.

As a result, the characteristics indicating the relationship between thesteering angle and the steering torque are offset in the direction inwhich the absolute value of the steering torque is increased, as shownin FIG. 20, and continuously changed from characteristic A tocharacteristic C as the deviation margin time is reduced. In this event,in order to maintain the steering angle, the steering torque needs to beincreased. If the steering torque is constant, the steering wheel 6 isgradually returned to the steering angle neutral position (point P₁ topoint P₂). Thus, the vehicle driving position can be prevented frombeing shifted to the right by a sudden turning operation by the driver.On the other hand, when the steering angle is maintained by the driver,the steering angle and the steering torque are moved from point P₁ topoint P₃. When the driver performs corrective steering from this state,the sign of the steering torque is not inverted until the vehicledriving position reaches the steering torque neutral position during aturning operation from the steering angle neutral position, since thesteering torque neutral position is offset to the turning side than thesteering angle neutral position in characteristic C. Therefore, thedriver can control the turning angle of the front wheels 5L and 5R justby reducing the steering torque and stopping the rotation of thesteering wheel 6 when the steering wheel 6 comes to the target angle.More specifically, in the reaction force offset control according to thedeviation margin time in Example 1, the direction of controlling theforce by the driver is not likely to be switched. Thus, the correctivesteering by the driver can be facilitated. As a result, the vehicledriving position becomes less likely to be overshot, and thus thecorrective steering amount can be reduced.

In the reaction force offset control according to the deviation margintime in Example 1, the shorter the deviation margin time, the larger theoffset amount. Thus, the steering torque neutral position is offset to aposition farther away from the steering angle neutral position with ashorter deviation margin time. When the driver performs correctivesteering to return the vehicle driving position to the vicinity of thecenter of the driving lane, the shorter the deviation margin time, thehigher the possibility of the vehicle driving position being close tothe white line. The closer to the white line, the more the turningoperation amount from the steering angle neutral position needs to beincreased. In this event, when the offset amount of the steering torqueneutral position relative to the steering angle neutral position issmall, there is a possibility that the steering torque exceeds theneutral position and the sign of the steering torque is inverted beforethe steering wheel comes to the target angle. Therefore, the steeringtorque can be prevented from exceeding the neutral position byincreasing the offset amount with a shorter distance to the white line.

[Effects Achieved by Combination of Reaction Force Offset ControlsAccording to Lateral Position and Deviation Margin Time]

In the steering reaction force control unit 20, the steering reactionforce torque offset section 36 selects the reaction force having alarger absolute value between the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position, as the steering reaction force torque offset amount.The adder 20 c adds the steering reaction force torque offset amount tothe steering reaction force torque. Thus, the steering reaction forcecharacteristics are offset in the direction in which the absolute valueof the steering reaction force torque is increased, according to thedeviation margin time or the lateral position.

In the reaction force offset control according to the deviation margintime, when the own vehicle is parallel to the white line, i.e., when theyaw angle is zero, the reaction force corresponding to the deviationmargin time is zero. For this reason, only a small reaction force can begenerated when the yaw angle is small even if the own vehicle is at aposition close to the white line. On the other hand, in the reactionforce offset control according to the lateral position, the reactionforce (reaction force corresponding to the lateral position) isgenerated in proportion to the distance to the white line. Thus, theshorter the distance to the white line, the larger the reaction forcecan be generated. Accordingly, the own vehicle can be more easilyreturned to the vicinity of the center of the driving lane.

On the other hand, in the reaction force offset control according to thelateral position, when the own vehicle is near the center of the drivinglane, the reaction force corresponding to the lateral position is zero.Therefore, the vehicle reaches the white line in a short time when theyaw angle is large and the vehicle speed is high even if the vehicle isnear the center of the driving lane. However, it is difficult toincrease the steering reaction force with good responsiveness. On theother hand, the reaction force offset control according to the deviationmargin time has characteristics that the reaction force (reaction forcecorresponding to the deviation margin time) is generated according tothe deviation margin time, and the reaction force rapidly rises when thedeviation margin time becomes 3 seconds or less. Thus, even when thevehicle reaches the white line in a short time, lane departure can besuppressed by increasing the steering reaction force with goodresponsiveness.

Therefore, by performing both the reaction force offset controlaccording to the deviation margin time and the reaction force offsetcontrol according to the lateral position, the lane departure can beeffectively suppressed while applying a stable reaction force accordingto the distance to the white line. In this event, an optimum steeringreaction force that is always required can be applied by using thereaction force having a larger absolute value between the reaction forcecorresponding to the deviation margin time and the reaction forcecorresponding to the lateral position.

As described above, Example 1 achieves the following effects.

(1) The steering control device includes: the steering unit 1 configuredto receive steering input from a driver; the turning unit 2 mechanicallyuncoupled from the steering unit 1 and configured to turn the left andright front wheels 5FL and 5FR; the turning control unit 19 configuredto control the turning amount of the turning unit 2 based on thesteering amount of the steering unit 1; the steering reaction forcecontrol unit 20 configured to set a steering reaction forcecharacteristic such that the larger the self-aligning torque, the largerthe steering reaction force, on coordinates with the self-aligningtorque and the steering reaction force as coordinate axes, and to applya steering reaction force corresponding to the self-aligning torque tothe steering unit 1 based on the steering reaction force characteristic;the lateral position calculation part 36 b configured to detect thelateral position of the own vehicle relative to the white line; and thesteering reaction force torque offset section 36 configured to offsetthe steering reaction force characteristic on the coordinates in such adirection that the closer the detected lateral position to the whiteline, the larger the absolute value of the steering reaction force.Thus, since the steering torque neutral position is offset more to theturning side than the steering angle neutral position, inversion of thesign of the steering torque is suppressed during the correctivesteering. As a result, the direction of controlling the force by thedriver becomes less likely to be switched. Therefore, the steering loadon the driver can be reduced.

(2) The steering reaction force characteristic is that the sign of thesteering reaction force is inverted over the position where theself-aligning torque is zero. Thus, since the steering torque neutralposition is offset more to the turning side than the steering angleneutral position, inversion of the sign of the steering torque issuppressed during the corrective steering. As a result, the direction ofcontrolling the force by the driver becomes less likely to be switched.Therefore, the steering load on the driver can be reduced.

(3) The steering control device further includes the deviation margintime selection part 39 d configured to calculate margin time that istime required for the own vehicle to reach the white line. The steeringreaction force torque offset section 36 calculates a lateral positionoffset amount such that the lateral position offset amount becomeslarger as the detected lateral position becomes closer to the whiteline, calculates a margin time offset amount as the margin time offsetamount becomes larger as the calculated margin time becomes shorter, andperforms offset using the larger offset amount between the lateralposition offset amount and the margin time offset amount. Thus, the lanedeparture can be effectively suppressed while applying a stable reactionforce according to the distance to the white line. In this event, anoptimum steering reaction force that is always required can be appliedby using the reaction force having a larger absolute value between thereaction force corresponding to the deviation margin time and thereaction force corresponding to the lateral position.

(4) The turning amount of the turning unit 2 is controlled according tothe steering amount of the steering unit 1 mechanically uncoupled fromthe turning unit 2. A steering reaction force characteristic in whichthe larger the self-aligning torque, the larger the steering reactionforce is set on coordinates with the self-aligning torque and thesteering reaction force as coordinate axes. When a steering reactionforce corresponding to the self-aligning torque is applied to thesteering unit 1 based on the steering reaction force characteristic, thesteering reaction force characteristic is offset on the coordinates insuch a direction that the closer the lateral position of the own vehicleto the white line, the larger the absolute value of the steeringreaction force. Thus, since the steering torque neutral position isoffset more to the turning side than the steering angle neutralposition, inversion of the sign of the steering torque is suppressedduring the corrective steering. As a result, the direction ofcontrolling the force by the driver becomes less likely to be switched.Therefore, the steering load on the driver can be reduced.

(5) The steering control device includes: the lateral positioncalculation part 36 b configured to detect the lateral position of theown vehicle relative to the white line; and the steering reaction forcecontrol unit 20 configured to control the turning amount of the turningunit 2 according to the steering amount of the steering unit 1mechanically uncoupled from the turning unit 2, to set a steeringreaction force characteristic such that the larger the self-aligningtorque, the larger the steering reaction force, on coordinates with theself-aligning torque and the steering reaction force as coordinate axes,and to offset the steering reaction force characteristic on thecoordinates in such a direction that the closer the detected lateralposition of the own vehicle to the white line, the larger the absolutevalue of the steering reaction force when applying a steering reactionforce corresponding to the self-aligning torque to the steering unit 1based on the steering reaction force characteristic. Thus, since thesteering torque neutral position is offset more to the turning side thanthe steering angle neutral position, inversion of the sign of thesteering torque is suppressed during the corrective steering. As aresult, the direction of controlling the force by the driver becomesless likely to be switched. Therefore, the steering load on the drivercan be reduced.

Example 2

FIG. 21 is a system diagram showing a steering system of a vehicleaccording to Example 2. Note that the same parts as those in Example 1are denoted by the same names and reference numerals, and descriptionthereof is omitted.

A steering device according to Example 2 mainly includes a steering unit1, a turning unit 2 and an EPS controller 25. In the steering device,the steering unit 1 configured to receive a steering input from a driveris mechanically coupled to the turning unit 2 configured to turn leftand right front wheels (turning wheels) 5FL and 5FR. The steering unit 1includes a steering wheel 6, a column shaft 7 and a torque sensor 26.The torque sensor 26 detects a steering torque of the driver inputted tothe column shaft 7 from the steering wheel 6. The turning unit 2includes a pinion shaft 11, a steering gear 12 and a power steeringmotor 27. The pinion shaft 11 is connected to the column shaft 7 througha torsion bar of the torque sensor 26. The power steering motor 27 is abrushless motor, for example, and has an output shaft coupled to a rackgear 15 through an unillustrated reducer. The power steering motor 27outputs an assist torque to assist the steering force of the driver to arack 16 in response to a command from the EPS controller 25.

In addition to the torque sensor 26, an image of a driving road in frontof an own vehicle shot by a camera 17 and a vehicle speed (vehicle bodyspeed) detected by a vehicle speed sensor 18 are inputted to the EPScontroller 25. The EPS controller 25 includes an assist torque controlunit 28 and an image processing unit 21. The assist torque control unit28 generates a command assist torque based on the input information, andoutputs the generated command assist torque to a current driver 29.

The current driver 29 controls a command current to the power steeringmotor 27 by torque feedback to cause an actual assist torque estimatedfrom a current value of the power steering motor 27 to coincide with thecommand assist torque. The image processing unit 21 recognizes whitelines (driving lane marking) on the left and right of a driving lane byperforming image processing, such as edge extraction, on the image ofthe driving road in front of the own vehicle shot by the camera 17.

[Assist Torque Control Unit]

FIG. 22 is a control block diagram of the assist torque control unit 28.An assist torque calculation section 41 calculates an assist torque byreferring to a preset assist torque map, based on the steering torqueand the vehicle speed. The assist torque characteristic in the assisttorque map has a characteristic such that the assist torque increaseswith an increase in the absolute value of the steering torque or with adecrease in the vehicle speed.

An assist torque offset section 42 calculates an assist torque offsetamount to offset the assist torque characteristic in the assist torqueoffset control according to the lateral position or deviation margintime, based on the vehicle speed and the image of the driving road infront of the own vehicle. The assist torque offset section 42 isdescribed in detail later. A subtractor 28 a outputs a value obtained bysubtracting the assist torque offset amount from the assist torque, as afinal command assist torque, to the current driver 29.

[Assist Torque Offset Section]

FIG. 23 is a control block diagram of the assist torque offset section42. A reaction force selection part 42 c selects a reaction force havinga larger absolute value between the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position, as the assist torque offset amount.

[Assist Torque Offset Control Operation According to Lateral Position]

In the assist torque offset control according to the lateral position,the reaction force corresponding to the lateral position is subtractedfrom the assist torque as the assist torque offset amount. Thus, theassist torque characteristic indicating the assist torque correspondingto the steering torque is offset in such a direction that the shorterthe distance to the white line, the smaller the absolute value of theassist torque, as shown in FIG. 24. Note that FIG. 24 shows the casewhere the vehicle is closer to the right lane. When the vehicle iscloser to the left lane, the assist torque characteristic is offset in adirection opposite to that shown in FIG. 24. Thus, as the characteristicrepresenting the relationship between the steering angle and thesteering torque, the characteristic shown in FIG. 20 of Example 1 isobtained. Thus, the same effect as that achieved by the reaction forceoffset control according to the lateral position in Example 1 can beachieved.

[Assist Torque Offset Control Operation According to Deviation MarginTime]

In the assist torque offset control according to the deviation margintime, the reaction force corresponding to the deviation margin time issubtracted from the assist torque as the assist torque offset amount.Thus, the assist torque characteristic indicating the assist torquecorresponding to the steering torque is offset in such a direction thatthe shorter the deviation margin time, the smaller the absolute value ofthe assist torque, as shown in FIG. 24. Note that FIG. 24 shows the casewhere the vehicle is closer to the right lane. When the vehicle iscloser to the left lane, the assist torque characteristic is offset in adirection opposite to that shown in FIG. 24.

Thus, as the characteristic representing the relationship between thesteering angle and the steering torque, the characteristic shown in FIG.20 of Example 1 is obtained. Thus, the same effect as that achieved bythe reaction force offset control according to the deviation margin timein Example 1 can be achieved.

Note that the effects achieved by performing both of the assist torqueoffset controls according to the lateral position and the deviationmargin time in Example 2 are also the same as those achieved byperforming both of the reaction force offset controls according to thelateral position and the deviation margin time in Example 1.

As described above, Example 2 achieves the following effects.

(6) The steering control device includes: the steering unit 1 configuredto receive steering input from a driver; the turning unit 2 mechanicallycoupled to the steering unit 1 and configured to turn the left and rightfront wheels 5FL and 5FR; the assist torque control unit 28 configuredto set an assist torque characteristic such that the larger the steeringtorque, the larger the assist torque, on coordinates with the steeringtorque and the assist torque as coordinate axes, and to apply an assisttorque corresponding to the steering torque to the turning unit 2 basedon the assist torque characteristic; the lateral position calculationpart 36 b configured to detect the lateral position of the own vehiclerelative to the white line; and the assist torque offset section 42configured to offset the assist torque characteristic on the coordinatesin such a direction that the closer the detected lateral position to thewhite line, the smaller the absolute value of the assist torque. Thus,since the steering torque neutral position is offset more to the turningside than the steering angle neutral position, inversion of the sign ofthe steering torque is suppressed during the corrective steering. As aresult, the direction of controlling the force by the driver becomesless likely to be switched. Therefore, the steering load on the drivercan be reduced.

(7) The assist torque characteristic is that the sign of the assisttorque is inverted over the position where the steering torque is zero.Thus, since the steering torque neutral position is offset more to theturning side than the steering angle neutral position, inversion of thesign of the steering torque is suppressed during the correctivesteering. As a result, the direction of controlling the force by thedriver becomes less likely to be switched. Therefore, the steering loadon the driver can be reduced.

(8) The steering control device further includes the lateral positioncalculation part 36 b configured to detect the lateral position of theown vehicle relative to the white line. The assist torque offset section42 calculates a lateral position offset amount such that the lateralposition offset amount becomes larger as the detected lateral positionbecomes closer to the white line, calculates a margin time offset amountsuch that the margin time offset amount becomes larger as the calculatedmargin time becomes shorter, and performs offset using the larger offsetamount between the lateral position offset amount and the margin timeoffset amount. Thus, the lane departure can be effectively suppressedwhile applying a stable reaction force according to the distance to thewhite line. In this event, an optimum steering reaction force that isalways required can be applied by using the reaction force having alarger absolute value between the reaction force corresponding to thedeviation margin time and the reaction force corresponding to thelateral position.

(9) An assist torque characteristic in which the larger the steeringtorque, the larger the assist torque is set on coordinates with thesteering torque and the assist torque as coordinate axes. When an assisttorque corresponding to the steering torque is applied to the steeringunit 1 mechanically coupled to the turning unit 2 based on the assisttorque characteristic, the assist torque characteristic is offset on thecoordinates in such a direction that the closer the lateral position ofthe own vehicle to the white line, the smaller the absolute value of thesteering torque. Thus, since the steering torque neutral position isoffset more to the turning side than the steering angle neutralposition, inversion of the sign of the steering torque is suppressedduring the corrective steering. As a result, the direction ofcontrolling the force by the driver becomes less likely to be switched.Therefore, the steering load on the driver can be reduced.

(10) The steering control device includes: the lateral positioncalculation part 36 b configured to detect the lateral position of theown vehicle relative to the white line; and the assist torque controlunit 28 configured to set an assist torque characteristic such that thelarger the steering torque, the larger the assist torque, on coordinateswith the steering torque and the assist torque as coordinate axes, andto offset the assist torque characteristic on the coordinates in such adirection that the closer the detected lateral position of the ownvehicle to the white line, the smaller the absolute value of thesteering torque when applying an assist torque corresponding to thesteering torque to the steering unit 1 mechanically coupled to theturning unit 2 based on the assist torque characteristic. Thus, sincethe steering torque neutral position is offset more to the turning sidethan the steering angle neutral position, inversion of the sign of thesteering torque is suppressed during the corrective steering. As aresult, the direction of controlling the force by the driver becomesless likely to be switched. Therefore, the steering load on the drivercan be reduced.

The invention claimed is:
 1. A steering control device comprising: asteering wheel configured to receive steering input from a driver; asteering gear mechanically uncoupled from the steering wheel andconfigured to turn a turning wheel; a sensor configured to detect alateral position of an own vehicle relative to a white line; and acontroller configured to control a turning amount of the steering gearbased on a steering amount of the steering wheel, to set a steeringcharacteristic such that a steering torque increases in accordance withan increase of a steering angle, and to offset the steeringcharacteristic in such a direction that the closer the detected lateralposition to the white line, the larger an absolute value of a steeringreaction force, when applying the steering reaction force to thesteering wheel based on the steering characteristic.
 2. The steeringcontrol device according to claim 1, wherein the steering characteristicis that a sign of the steering reaction force is inverted over aposition where a self-aligning torque is zero.
 3. The steering controldevice according to claim 1, wherein the controller is furtherconfigured to: calculate margin time that is time required for the ownvehicle to reach the white line, calculate a lateral position offsetamount such that the lateral position offset amount becomes larger asthe detected lateral position becomes closer to the white line,calculate a margin time offset amount such that the margin time offsetamount becomes larger as the calculated margin time becomes shorter, andperform offset using the larger offset amount between the lateralposition offset amount and the margin time offset amount.
 4. A steeringcontrol device comprising: a steering wheel configured to receivesteering input from a driver; a steering gear mechanically coupled tothe steering wheel and configured to turn a turning wheel; a sensorconfigured to detect a lateral position of an own vehicle relative to awhite line; and a controller configured to set an assist torquecharacteristic such that the larger a steering torque, the larger anassist torque, on coordinates with the steering torque and the assisttorque as coordinate axes, and to offset the assist torquecharacteristic on the coordinates in such a direction that the closerthe detected lateral position to the white line, the smaller an absolutevalue of the assist torque, when applying the assist torquecorresponding to the steering torque to the steering gear based on theassist torque characteristic.
 5. The steering control device accordingto claim 4, wherein the assist torque characteristic is that a sign ofthe assist torque is inverted over a position where the steering torqueis zero.
 6. The steering control device according to claim 4, whereinthe controller is further configured to: calculate margin time that istime required for the own vehicle to reach the white line, calculate alateral position offset amount such that the lateral position offsetamount becomes larger as the detected lateral position becomes closer tothe white line, calculate a margin time offset amount such that themargin time offset amount becomes larger as the calculated margin timebecomes shorter, and perform the offset using the larger offset amountbetween the lateral position offset amount and the margin time offsetamount.
 7. A steering control method comprising: controlling a turningamount of a steering gear according to a steering amount of a steeringwheel mechanically uncoupled from the steering gear; setting a steeringreaction force characteristic such that the larger a self-aligningtorque, the larger a steering reaction force, on coordinates with theself-aligning torque and the steering reaction force as coordinate axes;and offsetting the steering reaction force characteristic on thecoordinates in such a direction that the closer the lateral position ofan own vehicle to a white line, the larger an absolute value of thesteering reaction force, when applying the steering reaction forcecorresponding to the self-aligning torque to the steering wheel based onthe steering reaction force characteristic.
 8. A steering control devicecomprising: a sensor configured to detect a lateral position of an ownvehicle relative to a white line; and a controller configured to controla turning amount of a turning unit according to a steering amount of asteering unit mechanically uncoupled from the turning unit, to set asteering reaction force characteristic such that the larger aself-aligning torque, the larger a steering reaction force, oncoordinates with the self-aligning torque and the steering reactionforce as coordinate axes, and to offset the steering reaction forcecharacteristic on the coordinates in such a direction that the closerthe detected lateral position of the own vehicle to the white line, thelarger an absolute value of the steering reaction force, when applyingthe steering reaction force corresponding to the self-aligning torque tothe steering unit based on the steering reaction force characteristic.9. A steering control method comprising: setting an assist torquecharacteristic such that the larger a steering torque, the larger anassist torque, on coordinates with the steering torque and the assisttorque as coordinate axes; and offsetting the assist torquecharacteristic on the coordinates in such a direction that the closer alateral position of an own vehicle to a white line, the smaller anabsolute value of the steering torque, when applying the assist torquecorresponding to the steering torque to a steering wheel mechanicallycoupled to a steering gear based on the assist torque characteristic.10. A steering control device comprising: a sensor configured to detecta lateral position of an own vehicle relative to a white line; and acontroller configured to set an assist torque characteristic such thatthe larger a steering torque, the larger an assist torque, oncoordinates with the steering torque and the assist torque as coordinateaxes, and to offset the assist torque characteristic on the coordinatesin such a direction that the closer the detected lateral position of theown vehicle to the white line, the smaller an absolute value of thesteering torque, when applying the assist torque corresponding to thesteering torque to a steering unit mechanically coupled to a turningunit based on the assist torque characteristic.